Object-oriented in C? Implementing interfaces from scratch

This article discusses how to simulate the concept of interfaces in object-oriented programming in C language. We will take the calculation of vehicle prices as an example, implement them in Java and C languages ​​respectively, compare the differences between the two languages, and show how to implement the basic functions of the interface in C.

Java implementation:

In Java, interface is defined using the interface keyword, and classes implement interfaces through implements keyword. The sample code is as follows:

 <code class="java">interface Vehicle { int price(); } class Car implements Vehicle { private final int speed; public Car(int speed) { this.speed = speed; } @Override public int price() { return speed * 60; } } class Motorcycle implements Vehicle { private final int cc; public Motorcycle(int cc) { this.cc = cc; } @Override public int price() { return cc * 10; } } public class Main { public static void printVehiclePrice(Vehicle vehicle) { System.out.println("$" vehicle.price() ".00"); } public static void main(String[] args) { Car car = new Car(120); Motorcycle motorcycle = new Motorcycle(1000); printVehiclePrice(car); printVehiclePrice(motorcycle); } }</code>

C language implementation:

There is no direct interface mechanism in C language. We can simulate the behavior of an interface by enumerating types, structures, and function pointers.

First define the enumeration type to represent the vehicle type:

 <code class="c">typedef enum { VEHICLE_CAR, VEHICLE_MOTORCYCLE } VehicleType;</code>

Then define the vehicle structure, including type information and function pointers:

 <code class="c">typedef struct { VehicleType type; int (*price)(void*); // 函数指针，指向价格计算函数} Vehicle;</code>

Next, realize the structure of automobile and motorcycle:

 <code class="c">typedef struct { VehicleType type; int speed; } Car; typedef struct { VehicleType type; int cc; } Motorcycle;</code>

The corresponding initialization and price calculation functions:

 <code class="c">Car* car_init(int speed) { Car* car = malloc(sizeof(Car)); car->type = VEHICLE_CAR; car->speed = speed; return car; } int car_price(void* car) { return ((Car*)car)->speed * 60; } Motorcycle* motorcycle_init(int cc) { Motorcycle* motorcycle = malloc(sizeof(Motorcycle)); motorcycle->type = VEHICLE_MOTORCYCLE; motorcycle->cc = cc; return motorcycle; } int motorcycle_price(void* motorcycle) { return ((Motorcycle*)motorcycle)->cc * 10; }</code>

Finally, implement the vehicle_price function and call different price calculation functions according to the vehicle type:

 <code class="c">int vehicle_price(Vehicle* vehicle) { switch (vehicle->type) { case VEHICLE_CAR: return car_price((Car*)vehicle); case VEHICLE_MOTORCYCLE: return motorcycle_price((Motorcycle*)vehicle); default: return 0; } } void print_vehicle_price(Vehicle* vehicle) { printf("$%d.00\n", vehicle_price(vehicle)); } int main() { Vehicle car_v = {VEHICLE_CAR, car_price}; ((Car*)&car_v)->speed = 120; // 强制类型转换Vehicle motorcycle_v = {VEHICLE_MOTORCYCLE, motorcycle_price}; ((Motorcycle*)&motorcycle_v)->cc = 1000; // 强制类型转换print_vehicle_price(&car_v); print_vehicle_price(&motorcycle_v); free((Car*)&car_v); free((Motorcycle*)&motorcycle_v); return 0; }</code>

This C language implementation simulates the behavior of interfaces, but requires manual management of memory and type conversion, which is more complex than Java's interface mechanism. This approach improves the maintainability and readability of the code when dealing with complex data structures, such as abstract syntax trees (AST).

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